The goal of this research program is to understand how the circuits of the spinal cord contribute to coordinated actions of the musculoskeletal system. One way to make sense of spinal circuitry is to evaluate the component reflexes in terms of the mechanical actions, lengths and forces of the associated muscles. For example, the functions of reciprocal inhibition can best be appreciated by knowing that the linked muscles are mutually antagonistic and that the strength of the inhibition depends on muscle length. In general, proprioceptive feedback derives from a number of different receptors and converges and diverges extensively in the spinal cord. A major reason why the functions of these pathways are poorly understood is that information on the three-dimensional actions of muscles and neuromuscular compartments is lacking. Each muscle or compartment acts in a unique direction across one or more joints, and each is associated with a unique pattern of intermuscular reflexes. The purpose of this research is to establish, for the cat hindlimb, how the organization of spinal reflexes corresponds to the mechanical actions and natural activity patterns of the associated muscles. Mechanical actions will be determined by measuring the torque exerted by a number of muscles about the ankle, knee and hip. Transmission of forces through complex or distributed tendons will be measured by implanted transducers. Reflex linkages between muscles will be investigated in decerebrate cats by applying selective length inputs to one muscle or compartment and recording the force response in the other at different forces and lengths and states of activation. The resulting torque profiles and reflex maps, along with dynamic properties of the limb, will provide basic data for the formulation of models which can explain how the action of spinal circuits contributes to coordinated limb movement and why reflex organization is modified for certain motor tasks, such as locomotion and turning. The global picture of reflex organization resulting from these studies can potentially guide the formulation of analytical models of information processing in the spinal cord as well as provide a basis for new approaches to the rehabilitation of injury to the motor system. The study of force transmission in tendons may suggest improvements in tendon transfer surgery.